This invention relates to laser beam level instruments, of which a typical example is illustrated and described in U.S. Pat. Nos. 4,062,634 and 4,221,483 of Joseph F. Rando et al, issued to Spectra-Physics Inc. Such instruments sweep a laser beam in a horizontal plane, and are typically used in surveying and construction industries. They utilize a penta mirror or a penta prism (e.g. a penta assembly) which always directs the incoming beam through essentially a right angle, as long as the incoming beam is within the plane that contains the normal to each reflecting surface (hereinafter called the plane of the penta assembly). It has been desired, particularly in applications where the penta assembly is used in continuous rotation such as in surveying laser applications, to have the penta assembly reflect light through a right angle with an incoming beam that is out of the plane of the penta assembly.
In a typical surveying laser application, the input laser beam is directed upward (or downward) towards the first reflecting surface, e.g. a penta assembly mirror or a penta prism assembly. This penta assembly is typically supported to rotate by suitable bearing(s) and is fastened to the base or support of the aligning apparatus which is only approximately level, e.g within less than or equal to 5xc2x0. Although the structure is not perfectly level, the input beam is automatically compensated by another apparatus (not shown) so as to be precisely plumb, i.e. within some very small tolerance. Since the structure, and hence the penta assembly, is not precisely level and the compensating apparatus has set the vertical beam to be precisely plumb, some angle then exists between the vertically plumb laser beam and the plane of the penta assembly.
There are two configurations for a penta assembly, namely mirror or prism. One configuration utilizes an obtuse inclusive angle between the reflecting or mirror surfaces of 135xc2x0 and the other configuration (the most common and a more compact design) utilizes an included acute angle of 45xc2x0. This latter configuration is shown schematically in FIG. 1, labeled Prior Art, and embodies a penta assembly including mirror surfaces M1 and M2. For purposes of this description the more common configuration is shown, but the principles of the invention are applicable to the other configuration using an obtuse inclusive angle. It should also be understood that the acute 45xc2x0 angle is exemplary, but may vary according to a desired result.
To show the precise nature of the compensation problem in detail is graphically difficult, but the principle may be illustrated generally as shown in FIG. 2. For purposes of simplified explanation, consider the condition when the incoming beam B1 lies within the plane of the normal of mirror M1 and mirror M2. In the three dimensional view (FIG. 2) the incoming beam (typically a collimated beam as from a laser source) is directed upward onto mirror M1 and is reflected through 45xc2x0 towards mirror M2. These mirrors preferably are xe2x80x98first surfacexe2x80x99 mirrors. The beam is then reflected again through 45xc2x0 by mirror M2 and exits the penta assembly at 90xc2x0 from its original input direction. If the angle of the incoming beam B1 is changed within the plane of the penta assembly, then the outgoing beam B2 will again be at a right angle to the incoming beam. It is this fact that makes the penta assembly so useful in creating an optical plumb.
If an apparatus is used that always generates a plumb beam and lies within the plane of the penta assembly, then the output beam will always be normal to plumb and can be called level. Assuming a condition wherein input beam B1 is plumb and lies in the plane of the penta assembly on the optical axis, then it is necessary to consider what happens as input beam B1 is moved within a plane that contains the optical axis and is perpendicular to the plane of the penta assembly. As the beam moves in this latter plane away from the optical axis, the reflection from the first mirror M1 is directed at an equal angle away from the normal to the first mirror M1 (e.g. the principle of reflection). This reflected light does not describe a line on the second mirror M2, but actually describes an arc as shown in FIG. 2. This arc is magnified over the distances that the apparatus may be required to operate; thus the scanning output laser beam will not be precisely straight in some locations along the scan.
Thus, light reflected from the second mirror M2 (the output of the penta assembly) then describes an arc SKi (i.e. skew) in the viewing plane normal to the optical axis. It is this arcuate trace of the output beam, from the second mirror, that is undesirable since it is not straight, but slightly curved.
It is the principal object of this invention to provide a means to allow the penta assembly always to reflect the incoming beam through essentially a right angle regardless whether or not the incoming beam lies in the plane of the penta assembly. In other words, the purpose of this invention is to reduce or eliminate such curvature of the output beam trace as described above.
It is important to recognize where this skew originates. Since it is caused by the description of an arc on the second mirror M2, if an optical element or elements is devised to eliminate such arcuate trace, the skew can be eliminated. Recalling that when the beam moves within the plane of the penta assembly, the motion of the beam on the mirror surface M2 is in a line on that second mirror and in the plane of the penta assembly, it follows that no compensation is needed in this plane. But, compensation is required for any ray outside of the plane of the penta assembly.
The present invention provides such compensation by reducing the input beam to the penta assembly to a spot on the first mirror M1, then directing the reflection of such spot to the second mirror M2, then translating the reflected beam from mirror M2 as a collimated output beam from the penta assembly. This is accomplished using simple passive optical elements, e.g. convex lenses or optical equivalents, which are easily incorporated into a penta assembly.